Magnetic resonance imaging has been developed as an imaging technique adapted to obtain both images of anatomical features of human patients as well as some aspects of the functional activities of biological tissue. These images have medical diagnostic value in determining the state of the health of the tissue examined.
In a magnetic resonance imaging process, a patient is typically aligned to place the portion of the patient's anatomy to be examined in the imaging volume of the magnetic resonance imaging apparatus. Such a magnetic resonance imaging apparatus typically comprises a primary magnet for supplying a constant magnetic field (B0) which, by convention, is along the z-axis and is substantially homogeneous over the imaging volume and secondary magnets that can provide linear magnetic field gradients along each of three principal Cartesian axes in space (generally x, y, and z, or x1, x2 and x3, respectively). A magnetic field gradient (ΔB0/Δxi) refers to the variation of the field along the direction parallel to B0 with respect to each of the three principal Cartesian axes, xi. The apparatus also comprises one or more RF (radio frequency) coils which provide excitation and detection of the magnetic resonance imaging signal.
The use of the magnetic resonance imaging process with patients who have implanted medical assist devices; such as cardiac assist devices or implanted insulin pumps; often presents problems. As is known to those skilled in the art, implantable devices (such as implantable pulse generators, leads, cardioverters, defibrillators, and/or pacemakers) are sensitive to a variety of forms of electromagnetic interference (EMI) because these enumerated devices include sensing and logic systems that respond to low-level electrical signals emanating from the monitored tissue region of the patient. Since the sensing systems and conductive elements of these implantable devices are responsive to changes in local electromagnetic fields, the implanted devices are vulnerable to external sources of severe electromagnetic noise, and in particular, to electromagnetic fields emitted during the magnetic resonance imaging procedure. Thus, patients with implantable devices are generally advised not to undergo magnetic resonance imaging procedures.
Continuing with the example of shielding from magnetic resonance imaging interference, it is noted that magnetic resonance imaging procedures are the most widely applied medical imaging modality, with the exception of x-ray procedures. Significant advances occur daily in the magnetic resonance imaging field, expanding the potential for an even broader usage.
There are primarily three sources of voltage that could lead to the malfunction of an implantable device, during a magnetic resonance imaging procedure. First, a static magnetic field is generally applied across the entire patient to align proton spins. Static magnetic field strengths up to 7 Tesla for whole body human imaging are now in use for research purposes. The increase in field strength is directly proportional to the acquired signal to noise ratio (SNR) which results in enhanced magnetic resonance image resolution. Consequently, there is impetus to increase static field strengths, but with caution for patient safety. These higher field strengths are to be considered in the development of implantable devices.
It is noted that for image acquisition and determination of spatial coordinates, time-varying gradient magnetic fields of minimal strength are applied in comparison to the static field. The effects of the gradients are seen in their cycling of direction and polarity. With present day pulse sequence design and advances in magnetic resonance imaging hardware, it is not uncommon to reach magnetic gradient switching speeds of up to 50 Tesla/sec (this is for clinical procedures being used presently). Additionally, fast imaging techniques such as echo-planar imaging and turbo FLASH are in use more frequently in the clinic. Non-invasive magnetic resonance angiography uses rapid techniques almost exclusively on patients with cardiovascular disease.
Previous research evaluating the effects of magnetic resonance imaging on pacemaker function did not include these fast techniques. Therefore, the use of magnetic resonance imaging for clinical evaluation for individuals with implantable cardiac devices may be an issue of even greater significance. Rapid magnetic resonance imaging techniques use ultra-fast gradient magnetic fields. The polarities of these fields are switched at very high frequencies. This switching may damage implantable devices or cause them to malfunction.
Lastly, in magnetic resonance imaging, a pulsed RF field is applied for spatial selection of the aligned spins in a specimen during a magnetic resonance imaging procedure. USFDA regulations relative to the power limits of the RF fields are in terms of a specific absorption rate (SAR), which is generally expressed in units of watts per kilogram. These limits may not consider the effects on implantable devices, as the deleterious effects of transmission of RF fields in the magnetic resonance imaging system may no longer be the primary concern in their design parameters.
It is noted that an implanted device; such as a cardioverter, defibrillator, and/or pacemaker; is used to sustain a patient's life through the regulation of cardiac function. Hence, such patients would be barred from safely availing themselves of magnetic resonance imaging as a diagnostic tool unless their implanted device is effectively shielded from the strongest interference that could be expected from a conventional magnetic resonance imaging session.
Of particular concern is the interaction between a conventional magnetic resonance imaging session and leads that are utilized by the implantable devices. These leads can function as antenna and convey the voltage from the conventional magnetic resonance imaging session to the implanted device or to the tissue of the patient. In one instance, the implantable device may be damaged, thereby jeopardizing the ability to sustain the life of the patient. In the other instance, the tissue of the patient may be seriously injured by the conveyed voltage.
It has been proposed to utilize filters in the leads to block the damaging voltage from being conveyed along the lead. Such filters may contain inductors and capacitors. However, these filters can also interfere with the desired signals being communicated along the leads to and from the implantable device and the tissue region of interest.
For example, a filter on a defibrillator lead must be able to block the damaging voltage from the magnetic resonance imaging session, but also be able to provide a viable path for a large voltage pulse to defibrillate a patient's heart. Conventional filter leads have not reliably provided the desired blocking power of the damaging voltage from the magnetic resonance imaging session and still consistently provide a viable path for a large voltage pulse to defibrillate a patient's heart because these components breakdown after the first defibrillation pulse, thereby destroying the ability to block the damaging voltage from the magnetic resonance imaging session.
Therefore, it is desirable to provide a defibrillator lead that blocks the damaging voltage from the magnetic resonance imaging session. Moreover, it is desirable to provide a defibrillator lead that blocks the damaging voltage from the magnetic resonance imaging session and provides a viable path for a large voltage pulse to defibrillate a patient's heart.